Wedgehead pins are definitely in right now and the pinball price market is always going up and up. (to stupid levels)
Any EMs =/= Gottlieb wedgeheads in terms of collectibility.
Not sure where to post this, so I'll just put it here, and maybe eventually write up a better explanation and place it on the price check forums.
Clay's guide here explains it a lot better, but I'll place a TLDR version here. Clay's guide is: pinrepair.com and scroll down to evolution (gottlieb and williams).
By all means this is just my opinion. However, generally in the EM pinball community most people would agree with the general comments I'll place here. There could be some debate between a game or two, but most if not all of the desirability comments will be correct. I'll rate them in very basic categories of terrible, ok, good, and great games, going from worst to best.
"Why are some EMs worth more than others, and which ones are those?"
Any game with "impulse flippers" are terrible games. That starts with the first games with flippers in 1947 through early 50s. The reason is, you're just pushing a button and making a flipper swing momentarily. No control can really be had. That being said, some of the desirable games in this area are Humpty Dumpty and some of the fairy tale series by Gottlieb. (Lady Robin Hood, etc). Chicago Coin's Thing is a neat game as well. Most of these games fall in the $200-$1000 range for nice ones. They're on the cheaper side.
The first "fun" game is Spot Bowler by Gottlieb in 1951. If you find one of these, get ready to shell out thousands (yes, 3 zeros). It's a rare game, and it's a fun one to play. Other games in the 50s that are stellar are World Champ, Knockout, Hawaiian Beauties, Twin Bill. These are all games that will set you back $2000+ or more. Other games in this era usually go for ~$700ish plus and up. Most people will say, you really can't find a nice looking woodrail game for less than $2000. There are exceptions of course, but most are in that price range. (wood rails instead of the metal rails starting in the 60s).
Then come the Gottlieb Wedgeheads, and Williams Reverse Wedgeheads. Most, if not all of the community will agree that ANY gottlieb wedgehead are some of the greatest EMs ever made. There are exceptions of course, but not many. Ship Ahoy is, you guessed it, a wedgehead because of the cosmetic design of the head. These games started in the 60s and went through the last EMs in 1979. These are all single player games, where as the multiplayer games aren't nearly as desireable. The reason being is on a multiplayer game ALL of the features need to reset each ball, because there is no memory system. When playing a Single Player game, there is no reason to reset anything, so the rule sets are much deeper, think of a wizard mode on a later game. Something that is a general rule of thumb is single players are good, and multiplayers are not. There are a few gottlieb multiplayers that are very good (Quick Draw/Fast Draw, Jet Spin/Super Spin, and Big Brave/Big Indian). Some people also comment on Solar City/Target Alpha as well.
In the 60s and especially 70s, Bally, and Williams multiplayer games are mostly terrible. There is one exception to a 70s williams game which is Gulfstream (single player) which is a "good, but not great" game. Space mission (multiplayer) is an ok game, along with Aztec, Little Chief, and Grand Prix. However, Fantastic (multiplayer), Big Ben (single Player), swinger, Darling, and the lucky series, they are just horrible games. These are the games people are probably thinking of when they say "No em is worth more than $250" or "Em's aren't worth anything really". Bally games in the 70s aren't the greatest either except for 3. The 2 multiball games Fireball and 4 million BC are very good, Nip It isn't bad, and then Captain Fantastic isn't bad either.
The greatest add a ball games, all the add-a-ball collectors comment on are: Pop a Card, Majorettes, and, yes you guessed it again, Ship Ahoy. Ship Ahoy is a desireable gottlieb wedgehead add a ball game, hence mine and Otaku's comments on the price.
There are a lot of EMs out there, and some are absolutely fantastic, except Williams Fantastic.

You just have to find the right person, and usually, unfortunately, on these forums, there aren't a lot of EM savvy folks.
Hope this helps.
-Pat